Specific Imaging Findings. Cranial MRI findings in Langerhans cell histiocytosis (LCH) include: (1) lesions of the facial bones, skull base, and calvarium; (2) intracranial extra-axial lesions (hypothalamic–pituitary region, meninges, circumventricular organs); (3) intra-axial changes; and (4) cerebral atrophy. Intracranial findings are typically associated with central diabetes insipidus (DI) and characterized by lack of high signal intensity of the posterior pituitary gland on T1-weighted images (from loss of ADH storage granules) and thickened enhancing pituitary stalk, which measures over 3 and even 7 mm in diameter. However, the posterior pituitary bright spot may persist in DI patients; on the other hand, other disease processes may present in a similar fashion. Hypothalamic (along the floor of the third ventricle) involvement with contrast enhancement can be seen, while pituitary and optic chiasm infiltration are found in some cases. In a minority of patients the infundibulum may be very thin and thread-like, under 1 mm in diameter; it can even be normal on MRI. The pineal gland may also show mild enlargement and contrast enhancement. Progressive reduction in size of the anterior pituitary on MRI is associated with a higher risk of additional endocrine defects. Repeated MRI studies in DI patients are of limited value for assessing a response to therapy, but are important for monitoring bone lesions and possible parenchymal CNS disease. Pertinent Clinical Information. LCH mainly affects children between 2 and 5 years of age with widely varying clinical manifestations and it may also occur in adults. CNS is affected in around 16% of patients, and DI is the most common manifestation (in 25%), followed by GH deficiency (in 10%) – usually diagnosed years after DI. However, a number of other diseases can cause DI and 30–50% of cases are considered idiopathic. LCH is usually self-limited in the absence of organ dysfunction. Brain involvement appears to be the single most important factor in determining quality of life.
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